This invention relates, in general, to forming a cavity in a substrate, and more particularly, to a pressure sensor and its method of fabrication.
Semiconductor devices such as piezoresistive pressure sensors, accelerometers, mass flow sensors, and the like can utilize a thin semiconductor layer, or sensor diaphragm, as a sensing or active element. A piezoresistor is formed in the sensor diaphragm, and the resistance of the piezoresistor varies with the deformation of the sensor diaphragm. With the continuing device integration efforts to produce smaller and more compact semiconductor components, the die size of piezoresistive pressure sensors and the like must also be reduced.
One prior art method of shrinking the die size is depicted in FIG. 1 where a recess 108 of approximately 1-2 microns deep is first etched into a frontside surface 102 of a substrate 109. A substrate 111 is subsequently bonded to substrate 109 covering recess 108. Substrate 111 is thinned to form a diaphragm portion 104 of pressure sensor 112. A backside surface 103 of substrate 109 is then anisotropically etched with potassium hydroxide (KOH) to form a port 101 which extends from backside surface 103 to recess 108.
However, this prior art method has certain drawbacks. In particular, the shallow depth of recess 108 produces reliability problems. The reliability of pressure sensor 112 can be degraded from particles, fluids, and moisture trapped within recess 108. Furthermore, the backside anisotropic etch is a time consuming process which requires frontside protection, especially when additional semiconductor devices are fabricated on the frontside of pressure sensor 112.
Accordingly, a need exists for a method of shrinking the die size of a pressure sensor. The method should not degrade the reliability of the pressure sensor, should be compatible with fabricating additional semiconductor devices on the same chip as the pressure sensor, and should be compatible with piezoresistive, capacitive, and other sensing technologies.